Presently, myocardial ischemia, usually from the narrowing of the coronary arteries as a result of atherosclerosis, is one of the leading causes of cardiovascular disease-related deaths in the world. Clinically, myocardial ischemia involves a decreased oxygen and nutrient delivery to the myocardium resulting from diminished coronary artery blood flow which in turn leads primarily to abnormalities of left ventricular function and cardiac rhythm and the consequences thereof. Myocardial ischemia occurs when the demands of the heart for oxygen and nutrients are not met commensurately by available blood supply. The physiological effects of myocardial ischemia range from minimal to a complete failure of cardiac pumping function depending upon the degree of myocardial involvement and/or associated cardiac rhythm abnormalities. Clinical manifestations of myocardial ischemia include chest pain or discomfort (angina); respiratory distress, including shortness of breath; fatigue; reduced exercise capacity or tolerance; and nausea.
Several factors make the early diagnosis and prevention of myocardial ischemia, as well as the monitoring of the progression of myocardial ischemia, relatively difficult. First, the onset of myocardial ischemia is generally subtle and sometimes occurs without any clinical manifestations perceptible to the patient. Often, the symptoms are mild and ignored. The patient may also compensate by changing his or her daily activities in an unconscious manner to minimize symptoms. As a result, myocardial ischemia can remain undiagnosed until more serious problems arise, such as severe congestive heart failure leading to cardiac arrest or pulmonary edema. Moreover, the susceptibility to suffer from myocardial ischemia depends upon the patient's age, sex, physical condition, and other factors, such as diabetes, blood pressure, cholesterol and homocystine levels. No one factor is dispositive. Finally, annual or even monthly checkups provide, at best, a “snapshot” of patient wellness and the incremental and subtle clinicophysiological changes which portend the onset or progression of myocardial ischemia often go unnoticed, even with regular health care. Documentation of subtle changes following initiation of therapy, that can guide and refine further evaluation and therapy, can be equally elusive.
Nevertheless, taking advantage of frequently and regularly measured physiological measures, such as recorded manually by a patient, via an external monitoring or therapeutic device, or via implantable device technologies, can provide a degree of detection and prevention heretofore unknown. For instance, patients already suffering from some form of treatable heart disease often receive an implantable pulse generator (IPG), cardiovascular monitor, therapeutic device, or similar external wearable device, with which rhythm and structural problems of the heart can be monitored and treated. These types of devices are useful for detecting physiological changes in patient conditions through the retrieval and analysis of telemetered signals stored in an on-board, volatile memory. Typically, these devices can store more than thirty minutes of per heartbeat data recorded on a per heartbeat, binned average basis, or on a derived basis from, for example, a trial or ventricular electrical activity, ST and T wave electrocardiographic changes, coronary sinus blood flow and composition, cardiac enzyme release, minute ventilation, patient activity score, cardiac output score, mixed venous oxygen score, cardiovascular pressure measures, and the like. However, the proper analysis of retrieved telemetered signals requires detailed medical subspecialty knowledge, particularly by cardiologists.
Alternatively, these telemetered signals can be remotely collected and analyzed using an automated patient care system. One such system is described in a related, commonly owned U.S. Pat. No. 6,312,378, issued Nov. 6, 2001, the disclosure of which is incorporated herein by reference. A medical device adapted to be implanted in an individual patient records telemetered signals that are then retrieved on a regular, periodic basis using an interrogator or similar interfacing device. The telemetered signals are downloaded via an internetwork onto a network server on a regular, e.g., daily, basis and stored as sets of collected measures in a database along with other patient care records. The information is then analyzed in an automated fashion and feedback, which includes a patient status indicator, is provided to the patient.
While such an automated system can serve as a valuable tool in providing remote patient care, an approach to systematically correlating and analyzing the raw collected telemetered signals, as well as manually collected physiological measures, through applied cardiovascular medical knowledge to accurately diagnose the onset of a particular medical condition, such as myocardial ischemia, is needed. One automated patient care system directed to a patient-specific monitoring function is described in U.S. Pat. No. 5,113,869 ('869) to Nappholz et al. The '869 patent discloses an implantable, programmable electrocardiography (ECG) patient monitoring device that senses and analyzes ECG signals to detect ECG and physiological signal characteristics predictive of malignant cardiac arrhythmias. The monitoring device can communicate a warning signal to an external device when arrhythmias are predicted. Like the ECG morphology of malignant cardiac tachycardias, the electrocardiographic diagnosis of myocardial ischemia is well established and can be readily predicted using on-board signal detection techniques. However, the Nappholz device is limited to detecting tachycardias. The Nappholz device is patient specific and is unable to automatically take into consideration a broader patient or peer group history for reference to detect and consider the progression or improvement of myocardial ischemia. In addition, the Nappholz device does not take into account other physiological or chemical measures indicative of myocardial ischemia. Moreover, the Nappholz device has a limited capability to automatically self-reference multiple data points in time and cannot detect disease regression even in the individual patient. Also, the Nappholz device must be implanted and cannot function as an external monitor. Finally, the Nappholz device is incapable of tracking the cardiovascular and cardiopulmonary consequences of any rhythm disorder.
Consequently, there is a need for a systematic approach to detecting trends in regularly collected physiological and chemical data indicative of the onset, progression, regression, or status quo of myocardial ischemia diagnosed and monitored using an automated, remote patient care system. The physiological data could be telemetered signals data recorded either by an external or an implantable medical device or, alternatively, individual measures collected through manual means. Preferably, such an approach would be capable of diagnosing both myocardial ischemia conditions, as well as the symptoms of other diseases. In addition, findings from individual, peer group, and general population patient care records could be integrated into continuous, on-going monitoring and analysis.